The Centers for Disease Control and Prevention (CDC) confirmed on Tuesday that a patient being treated at a Dallas hospital has tested positive for Ebola, the first case diagnosed in the United States.

The patient left Liberia on September 19 and arrived in the United States on September 20, CDC director, Dr. Tom Frieden told reporters at a press conference Tuesday. It’s the first patient to be diagnosed with this particular strain of Ebola outside of Africa.

“[The patient] had no symptoms when departing Liberia or entering this country. But four or five days later on the 24th of September, he began to develop symptoms,” said Frieden.

The patient, who was in the U.S. visiting family in Texas, initially sought care on September 26, but was sent home and was not admitted until two days later. He was placed in isolation at Texas Health Presbyterian Hospital of Dallas, where he remains critically ill, according to Frieden.

“The next steps are basically threefold,” said Frieden. “First, to care for the patient … to provide the most effective care possible as safely as possible to keep to an absolute minimum the likelihood or possibility that anyone would become affected, and second, to maximize the chances that the patient might recover,” said Frieden.

Frieden said the CDC and Texas health officials were working to identify and monitor anyone who may have come in contact with the patient.

“It’s only someone who’s sick with Ebola who can spread the disease,” said Frieden. “Once those contacts are all identified, they’re all monitored for 21 days after exposure to see if they develop a fever.”

Frieden added that while it is possible that someone who had contact with the patient could develop Ebola in the coming weeks, he has no doubt the infection will be contained. At this point, he said, there is zero risk of transmission to anyone on the flight with the patient because he was not showing any symptoms at the time of travel.

It’s unclear how the patient became infected, but health officials said he “undoubtedly had close contact with someone who was sick with Ebola or who had died from it.”

The patient will stay at Texas Health Presbyterian Hospital for treatment, where epidemiologist Dr. Edward Goodman, said medical staff have a plan in place for some time now in the event that a traveler brought Ebola to the United States, noting that the team had a crisis preparedness meeting just one week before the patient arrived at the facility.

Hospital officials are currently evaluating different treatment options, including experimental therapies which have been successful in other patients, according to Frieden.

Both the CDC and the Texas Department of State Health Services performed lab testing that is said to be highly accurate for detecting the Ebola virus disease.

“Our lab has a specially trained team to handle high-risk patients like this,” said Dr. David Lakey, commissioner of Texas Department of State Health Services. “We have no other suspected cases in the state of Texas at this time.”

Zachary Thompson, director of Dallas County Health and Human Services urged residents to rest assured the agency was doing everything they could to ensure the safety of the community, and that they would be working closely with the CDC and local health officials to follow up and track possible contacts of the patient.

Texas Health Presbyterian Hospital of Dallas officials said in a statement Monday that an unnamed patient was being tested for Ebola and had been placed in “strict isolation” due to the patient’s symptoms and recent travel history, and that the facility was taking measures to keep its doctors, staff and patients safe.

The hospital had announced a day earlier that the patient’s symptoms and recent travel indicated a case of Ebola, the virus that has killed more than 3,000 people across West Africa and infected a handful of Americans who have traveled to that region.

The CDC has said 12 other people in the U.S. have been tested for Ebola since July 27. Those tests came back negative.

Four American aid workers who have become infected while volunteering in West Africa have been treated in special isolation facilities in hospitals in Atlanta and Nebraska, and a U.S. doctor exposed to the virus in Sierra Leone is under observation in a similar facility at the National Institutes of Health.

The U.S. has only four such isolation units but the CDC has insisted that any hospital can safely care for someone with Ebola.

According to the CDC, Ebola symptoms can include fever, muscle pain, vomiting and bleeding, and can appear as long as 21 days after exposure to the virus.

Jason McDonald, spokesman for the CDC, said health officials use two primary guidelines when deciding whether to test a person for the virus.

“The first and foremost determinant is have they traveled to the region (of West Africa),” he said. The second is whether there’s been proximity to family, friends or others who’ve been exposed, he said.